1. Field of the Invention
The present invention relates to a C-axis driving system in a machine tool having a spindle rotably supported on a headstock and for rotationally indexing and positioning the spindle to a specified rotational angle.
2. Discussion of the Related Art
For example, some numerically controlled (NC) lathes are equipped with a C-axis driving system which allows C-axis machining by a rotating tool to be performed while controlling the rotation of the spindle that is rotationally driven by a spindle driving motor. See, for example, Japanese Patent Publication No. 2566543, Japanese Patent Laid-Open Publication SHO 59-53140 and Japanese Utility Model Laid-open Publication HEI 1-114257.
Conventionally, this type of C-axis driving system has been provided in the following structure. As shown in FIG. 10, a worm shaft 82 engageable with a worm wheel 80 fixedly mounted on a spindle (unshown) is placed perpendicular to the axis of the spindle and supported by a unit casing 81. A pivotal shaft 83 generally equal in length to the worm shaft 82 is inserted into the unit casing 81 parallel to the worm shaft 82. The unit casing 81 is driven by a drive mechanism (not shown) so as to rotate about the pivotal shaft 83 between an engagement position for engagement with the worm wheel 80 and a disengagement position for disengagement therefrom. Further, a C-axis driving motor 84 is connected to an end portion of the worm shaft 82 of the unit casing 81.
When it is in the engagement position, the worm shaft 82 is rotatingly driven by the C-axis driving motor 84. Thus the spindle is rotationally indexed and positioned into a specified rotational angle via the worm wheel 80. In this state, milling or other rotating work can be performed. Otherwise for turning work, the worm shaft 82 is moved into the disengagement position.
In this connection, in order to obtain smooth engagement between the worm wheel 80 and the worm shaft 82, the rotational angle of the unit casing 81 is preferably as small as possible. From the viewpoint of making the rotational angle small, in some cases the center distance xe2x80x9cdxe2x80x9d between the worm shaft 82 and the pivotal shaft 83 is set to a large distance.
However, when the center distance xe2x80x9cdxe2x80x9d between the worm shaft and the pivotal shaft is set to a large distance as in the conventional systems, the entire C-axis unit increases in size thereby causing the structure to be complicated and the cost increases.
The present invention has been made considering the above problems and circumstances in the prior art.
An object of the invention is to provide a C-axis driving system for machine tools that is capable of maintaining a smaller sized C-axis unit as well as maintaining the costs while using a C-axis driving system that implements a smooth engagement with the worm wheel.
In order to achieve this and other objects according to the invention, a C-axis driving system for machine tools is provided that includes a worm wheel mounted on a spindle rotatably supported by a headstock. The C-axis driving system also includes a worm shaft with a worm formed therein that is directed perpendicular to an axis of the spindle and is provided so as to be pivotable about a pivotal shaft between an engagement position, where the worm shaft is engaged with the worm wheel and a disengagement position, where the worm shaft is disengaged therefrom. The C-axis driving system also includes a C-axis driving motor connected to the worm shaft that serves to rotationally index the spindle to a specified rotational angle. A worm shaft base rotatably supports the worm shaft. The pivotal shaft is provided on the worm shaft base so as to be directed perpendicular to an axis of the worm shaft and the pivotal shaft is supported by the headstock.
According to another aspect of the invention, in the C-axis driving system as described above, the C-axis driving motor is connected in series to an end portion of the worm shaft of the worm shaft base. Also the pivotal shaft is placed in proximity to the C-axis driving motor between the worm of the worm shaft and the C-axis driving motor.
With the C-axis driving system of the invention, the worm shaft is supported by the worm shaft base, while the pivotal shaft is placed on the worm shaft base so as to be directed perpendicular to the axis of the worm shaft. Therefore, the worm shaft base can be of a size that allows the worm shaft to be housed therein and supported, thus allowing the unit to be smaller in size when compared to the conventional systems where the worm shaft and the pivotal shaft are placed apart with a distance therebetween. This thereby allows the C-axis unit as a whole to be reduced in size.
Also, since the worm shaft base can be downsized, the structure can be simplified when compared with the conventional unit casing. This also results in reductions in the parts count and the cost.
Furthermore, when placing the pivotal shaft on the worm shaft base so that the rotational angle of the worm shaft base becomes small, a smooth engagement with the worm wheel can be achieved.
According to another aspect of the invention, the C-axis driving motor is connected in series at an end portion of the worm shaft of the worm shaft base. Also the pivotal shaft is provided in proximity to the driving motor between the worm and the C-axis driving motor. Therefore, while a smooth engagement with the worm wheel is ensured, the driving force can be reduced when compared to the conventional case where the whole unit including the C-axis driving motor is pivoted. Thus, the structure can be further simplified and the cost can be further reduced.